Local anesthesia is, in many cases, a superior means of pain control in avoiding the side effects of narcotic pain medication. Nerve blocks are commonly used for postoperative pain control as well as the primary anesthetic when the avoidance of general anesthesia is desired. The use of local anesthesia in this application renders an area insensitive to pain without affecting consciousness, speeding the detection of complications and patient recovery. However, when performing a nerve block, relatively large drug doses can be required to inhibit the nerve conduction to the surgical site. By way of example, injection volumes can be in the range of 20-40 cc. Unfortunately, as little as 1 cc of local anesthesia injected into an artery can cause seizure, and large intravascular local anesthesia injection will block the conduction system of the heart, possibly leading to cardiac arrest and death. Symptoms evidencing the onset of toxic anesthesia contamination include numbness of the tongue, lightheadedness and visual and auditory disturbances, and can occur at concentrations below 7.5 mcg/ml. Approximately 5 in 10,000 surgeries report severe complications caused by intravascular local anesthesia, but thousands of regional blocks are done every day.
There are very few tools available to anesthesiologists to protect patients from this problem. One known method involves an epinephrine marked ‘test dose’ of local anesthesia. During injection doctors watch for an increase in heart rate or blood pressure indicating an inadvertent intravascular injection. Unfortunately, this method is very unreliable. Those taking beta blockers or wearing a pacemaker may be unresponsive to epinephrine. Children often have blocks done under general anesthesia which alters the body's response to adrenaline and makes it difficult to determine if the local anesthesia is intravascular. An epinephrine marker during an epidural can cause complications by decreasing blood flow to the fetus in pregnant women. There remains, therefore, a continuing need for improved systems and methods for providing accurate and early warning of intravascular local anesthesia.